Charles M. Rick

Genetics and breeding

Amongst flowering plants the tomato is exceptionally well endowed for genetic and cytogenetic research. Its foremost attribute for such purposes, despite a relatively high haploid chromosome number (12), is its basic diploid nature. (Rick, 1971a). Abundant interplay between tomato genetics and tomato breeding has resulted in great mutual benefit and much research has bridged both areas. Many useful materials in tomato genetics have been contributed by tomato breeders, and conversely, as detailed in this chapter, studies in basic tomato genetics have permitted much progress in tomato breeding.

Cytogenetics of the Tomato

Publisher Summary Studies on the genetics of the tomato have been stimulated recently by the organization of the Tomato Genetics Cooperative, which, like the older and exemplary groups in maize, drosophila, and other organisms, serves to coordinate and facilitate the activities of many workers by offering a medium of exchange of stocks and information and proposing standard procedures wherever advisable. This review is concerned with the genetics and cytology of the tomato, lycopersicon esculentum, a widely cultivated annual species of the Solanaceae. Studies of other species of lycopersicon are also included insofar as they deal directly with lycopersicon esculentum .

Genetic variation inLycopersicon pimpinellifolium: Evidence of evolutionary change in mating systems

L. pimpinellifolium is a highly heterogeneous species, exhibiting pronounced trends from one end of its linear distribution to the other in nearly every studied genetic locus. Drastic differences between populations were also detected in genetic variability and rates of outcrossing. Highly significant positive correlations exist in every possible comparison between flower size, degree of stigma exsertion, heterozygosity, and allelic polymorphism. The hypothesis most compatible with observations proposes that the very uniform, highly self-pollinated biotypes originated from the more primitive, more variable, facultatively allogamous forms.

Isozymic gene linkage map of the tomato: Applications in genetics and breeding

SummaryNew linkage data are presented for the situation of five previously unlocated isozymic loci of the tomato and closely related species with homosequential chromosomes.Prx-1 lies on chromosome 1, where it is also linked withSkdh-1; Aps-2 is linked withGot-4 on chromosome 8;Tpi-2 has been allocated to chromosome 4; and a linkage has been detected betweenPgi-1 andEst-4, whose respective chromosome has not yet been determined. These and previously published data have been summarized in the form of an isozyme linkage map. Twenty-two loci have thus been mapped on nine of the twelve tomato chromosomes. We discuss some new applications of mapped isozymic genes. In certain types of segregations, isozymic genes are far more efficient than morphological markers in providing linkage information. They greatly expedite the cytogenetic investigation of species hybrids and can be utilized to facilitate backcross transfers of genes from wild to cultivated taxa.

Potential genetic resources in tomato species: clues from observations in native habitats.

Interspecific hybridization is playing an increasingly important role in the breeding of improved cultivars of higher plants. Lycopersicon is a good example of a genus in which the cultivated species (L. esculentum Mill.) is being improved in this fashion. The advantages offered by the tomato species for this purpose are: a. All species can be readily grown for experimental purposes, and L. esculentum is widely cultivated under a wide range of environmental conditions. b. Excellent sources of germ plasm now exist in the wild species as well as in modern and primitive cultivars of L. esculentum. c. All of the wild species can be hybridized with L. esculentumn, albeit requiring special aids in certain combinations; fertility and viability of the hybrid generations permit the intended gene transfers. All species have 12 pairs of chromosomes, which are essentially homologous. d. The cultivated species is well known genetically; its chromosomes have been mapped cytologically and genetically; it behaves as a basic diploid (27, 28).

Cytogenetic analysis of the tomato genome by means of induced deficiencies

Cytological studies of 74 deficiencies of tomato chromosomes induced by radiation and identified by the pseudo-dominant technique reveal the loci of 35 genes on 18 of the 24 arms of the complement. These findings integrated with data obtained from various trisomic types establish centromere positions, orientation of linkage groups, and markers on all but three of the arms. The prospects of obtaining a specific kind of deficiency for a given region were found to depend on : (1) kind of radiation applied, (2) (non-random) breakage frequency in different parts of the chromosome, (3) stability of broken ends, (4) tolerance of deficiency in different parts of the genome, and (5) relative vigor of the mutant homozygote used to detect the deficiency. Aspects of the frequently observed non-homologous pairing phenomenon are presented and discussed. Marker genes whose loci are known appear to be non-randomly distributed between and within chromosomes. Chromosome exchanges as determined by genetic crossing over and cytologically observed chiasmata are likewise non-randomly distributed between and within chromosomes.

GALAPAGOS TOMATOES AND TORTOISES

One of the several unique features of tomatoes native to the Galapagos IslandsLycopersicon esculentum var. minor (L. cheesmanii var. minor) and related formsis seed dormancy. Less than one per cent of their seeds, which are notably smaller than those of other known tomatoes, will germinate without treatment. Germination of up to ten per cent can be obtained with var. minor if part of the seed coat is carefully excised (Rick, 1956). The present report deals with attempts to break the seed dormancy and, particularly, to find mechanisms in the native environment that might account for germination and establishment there of the Galapagos tomatoes. Recently, improved germination of these forms was sought by trying methods that are familiar to horticulturists for their effectiveness in breaking seed dormancies of various plant species. We found that soaking seeds for various periods in stagnant or wellaerated water, subjection of dry or wet seeds to various temperatures or combinations of temperatures, and acid treatments were ineffective in raising emergence percentages. Following the discovery by Benedict and Robinson (1946) of the improved germination of guayule seeds effected by sodium hypochlorite treatments and successful applications of this method by Taylor (1949) to celery and by Laude (1951) to smilo grass, we tested its effectiveness on seeds of LA166, one of the most refractory of our Galapagos collections. Four lots were started: (1) control; (2) seed coats partly excised; (3) one-hour soaking in 2.6% sodium hypochlorite solution( half-strength household bleach); (4) soaking for two hours in the same strength solution. Seeds

Evolution of mating systems inLycopersicon hirsutum as deduced from genetic variation in electrophoretic and morphological characters

Populations in the central part of the distribution are mostly self-incompatible and tend to be highly variable for allozymic and morphological characters; those in the north and south limits are entirely self-compatible and tend to be genetically highly uniform. Gradations in variability are observed in the intermediate regions. Flower size tends to diminish in the peripheral areas. The extensive differences in genotype observed between the north and south marginal populations are not compatible with the concept of a single origin of self-compatibility, but suggest, along with other evidence, that the substitution of different alleles resulted from differentiation in the marginal areas from older, self-incompatible stocks of the central region. The conclusions regarding patterns of genetic variation and nature of evolution of mating systems inL. hirsutum conform to a remarkable extent with those reached previously forL. pimpinellifolium, a species that is distinct in morphology and ecological preferences yet has a similar latitudinal distribution.

Rates of cross-pollination inLycopersicon pimpinellifolium: Impact of genetic variation in floral characters

Cross-pollination was tested between different genotypes ofLycopersicon pimpinellifolium in an experimental plot situated in the southerly range of the species. Rates of cross-pollination were found to be highly correlated with flower size and were related to a lesser extent with degree of stigma exsertion. Comparisons of fertility suggest that exsertion of 1.5 mm or more from the anther tube orifice interferes with automatic self-pollination, thereby increasing rates of cross-pollination. A single species ofExomalopsis (Anthophoridae), observed to visit all tested genotypes ofL. pimpinellifolium, was responsible for most of the pollination. Other species of solitary bees and other bee families participated in cross-pollination.

The effect of isozyme selection on metric characters in an interspecific backcross of tomato — basis of an early screening procedure

SummaryThe extent of correlation was estimated between isozyme genotypes and the four widely segregating characters — leaf segment W/L ratio, stigma exsertion, fruit weight, and seed weight — in the first backcross of F1Lycopersicon esculentum x Solanum pennellii to the former parent. The inbred parents differ in their alleles at the 12 tested isozymic loci, which are known to mark a minimum of eight of the twelve tomato chromosomes. Based on the isozyme data, a mean heterozygosity value, ¯H, was calculated which estimates the proportion of pennillii alleles in each individual. Correlations between mean heterozygosity and observed levels of each quantitative trait were highly significant and positive or negative as expected from the relative parental values. Plants with the lowest mean heterozygosity — i.e., closest to the esculentum zymotype also had mean values closest to those of this parent amongst the whole backcross population for each of the quantitative traits.Bivariate and multiple regression analysis was used to evaluate the ability of isozymes vs diagnostic morphological characters to estimate the portion of recurrent parent genes carried in each backcross individual. The results suggest that isozyme data gives better estimates than single diagnostic morphological characters and approach the level obtained by combinations of three morphological traits. Since electrophoretic determinations are made on small seedlings, selection at that stage can effect great savings of space and effort by greatly deminishing the size of the population needed at maturity. As such, isozyme selection would precede morphological selection but not replace it, thus the predictive value of these biochemical markers as well as diagnostic morphological characters could be obtained.